This invention relates to a focussing lens servo driving device in an optical-type information reading device and, more particularly, to an irradiation light focussing lens servo driving device for use in such an optical-type information reading device.
In an optical-type video disk, so-called "pits" are concentrically or spirally formed in the surface of a recording disk. Video information is recorded by varying the length and interval of these pits. A reflecting film, produced by vacuum-evaporating aluminum or the like, is formed on the surface of the disk where the pits are formed to increase the light reflection factor.
In reading the information recorded on the disk, light is emitted onto the disk from above the surface of the disk where no pits are provided and the light reflected from the reflection film, which is modulated by the presence and absence of the pits, is subsequently demodulated.
A recording disk reading device of this type is provided with a lens focussing servo device which automatically adjusts the lens so that the incident light is focussed on the information recording surface of the disk at all times.
FIG. 1 illustrates schematically one example of a servo focussing device. In the device, light emitted from a light source such as a helium neon laser or the like passes through a collimator lens 2, a beam splitter 3 and a movable mirror 4 and is focussed near an information recording surface 7 by means of a focussing lens 5. A disk 6 is rotated at high speed by an electric motor 14. The reflected light is advanced back along the light path and it is split by the beam splitter 3 and is then converted into an electrical signal by a photo-electric conversion element.
It is impossible to make the disk 6 completely flat and, in general, the disk 6 is inclined when installed on the shaft of the motor 14. Accordingly, the recording surface 7 moves up and down as the disk 6 is rotated. Thus, in order to correctly read the information, it is necessary to move the focussing lens 5 up and down following the up and down movement of the recording surface 7 to thereby maintain the focus of the light beam very near the recording surface 7. For this purpose, a cylindrical lens 8 is disposed before the point where the reflected light from the disk is focussed by the lens 5, that is between the lens 5 and the conjugate point of the disk with respect to the lens 5 and a light receiving element 9 is arranged behind the cylindrical lens 8. The light receiving element 9 is divided into four light receiving element surfaces a, b, c and d as shown in FIG. 2. The light receiving element 9 is so arranged that the division lines of the light receiving element 9 form 45 degree angles with the central axis of the cylinder of the cylindrical lens 8. The light beam passing through the lens 8 is focussed on the two focal lines which are in the plane including the generatrix of the cylindrical lens 8 and in the plane perpendicular to the firstly-mentioned plane, respectively. By utilizing this nature, the configuration of the light beam projected onto the four light receiving element surfaces a, b, c and d are detected to determine the relation between the recording surface 7 and the focal position of the focussing lens 5.
That is, the light receiving surface of the light receiving element 9 is arranged at the position where the reflected light passing through the cylindrical lens 8 becomes substantially circular when the focussing point of the incident light is positioned precisely on the recording surface of the disk by the lens 5, as shown in FIG. 3B. Under this condition, the outputs Va, Vb, Vc and Vd of the light receiving units a, b, c and d are equal to one another. EQU Va+Vb=Vc+Vd
Accordingly, the output voltage V of a differential amplifier 10 receiving the outputs (Va+Vb) and (Vc+Vd) is zero. As a result, the outputs of an amplifier 11 and a lens drive device 12 are zero and therefore the position of the lens 5 is maintained unchanged.
In the case in which, as shown in FIG. 4A, the incident light is focussed behind the recording surface 7, that is, the distance between the recording surface 7 and the focussing lens 5 is shortened, the configuration of the light beam on the light receiving surface of the light receiving element 9 is as shown in FIG. 4B. EQU Va+Vb&gt;Vc+Vd
Therefore, the output voltage V(A) of the differential amplifier 10 is positive (V(A)&gt;0).
In the case in which the incident light is focussed before the recording surface 7 as shown in FIG. 5A, the configuration of the light beam is as shown in FIG. 5B EQU Va+Vb&lt;Vc+Vd
Therefore, the output V(A) of the differential amplifier 10 is negative (V(A)&lt;0).
Thus, if the distance between the lens and the recording surface of the disk is represented by D and the distance when the incident light is focussed correctly on the recording surface is represented by Dj, then the output V(A) of the differential amplifier 10 has an S-shaped waveform characteristic when plotted against D as illustrated in FIG. 6. Therefore, if the output V(A) is amplified as an error signal by the amplifier 11, and is then converted into a displacement value by the drive device 12 and used to control the position of the focussing lens 5 with the aid of the holder 13 automatic focus control is realized.
In general, before the information reading operation is started, the focusing lens 5 is positioned at its fartherst distance away from the recording surface of the disk. If, under this condition, the servo-operation is commenced with the servo loop of the servo focussing circuit closed, an error signal deviated from the linear part of the S-shaped characteristic curve shown in FIG. 6 is produced because the distance D between the lens and the recording surface is long. Therefore, in this case, it is difficult for the servo circuit to carry out an accurate control operation. Even if the servo circuit could operate, the focussing lens is driven out of the control operation range and the operation of the servo circuit is unstable, unless the speed of movement of the focussing lens, that is the value of dD/dt with respect to the disk, is low.